1. Field of the Invention
The invention relates to a method for bonding graphite to metal, and more particularly to such a method using an active alloy brazing process.
2. Description of the Related Art
Graphite has various superior properties, for instance, high thermal conductivity, low thermal expansion coefficient, low secondary electron emission coefficient and high emissivity. Thus, it has been researched so far to apply the graphite, for instance, to walls of a plasma container for a nuclear fusion reactor, a collector electrode of an electron tube, and capillaries of an ion laser tube. In these applications, a member made of graphite is supported by or secured to other members made of material other than graphite, in almost all cases, metal, by any means. When high mechanical strength and high thermal conductivity are required for such support and securing, a brazing process is usually chosen.
As is understood from the fact that the graphite is suitable for walls of a metal fusion furnace and a crucible, the graphite has an inherent property of not reacting with molten metal. However, the graphite reacts on an active metal such as Ti, Zr, Ta and Mo at a temperature over 1,000 degrees centigrade, and thus forms a layer composed of a carbide of these metals at an interface thereof, to thereby firmly bond with the metal.
For instance, Japanese Unexamined Patent Public Disclosure No. 62-21768 published on Jan. 30, 1987 teaches that graphite can be bonded to Mo or Ni--Cr--Fe alloy, including Mo by more than 8%, in an inert gas atmosphere, for instance, under heat treatment at a temperature of 1,400 degrees centigrade for 90 hours.
Japanese Unexamined Patent Public Disclosure No. 63-310778 published on Dec. 19, 1988 discloses a process in which a surface of a member composed of graphite is first metalized with an active brazing alloy having Ti--Cu--Ag structure, and then brazed with a brazing alloy such as Ag, Au and Ni, to thereby form a stress relaxation layer for bonding the graphite to metal.
In the aforementioned brazing processes in which an active alloy brazing material is to be used, since the active alloy reacts on and thereby bonds to the graphite only within the area of contact between the active alloy and the graphite surface sufficient pressure, for instance, in the range of 200 grams per square centimeter or more and also a temperature of approximately 1,000 degrees centigrade it is necessary for bonding to apply to the contact area. In addition, since the reaction develops at relatively slow speed under the high temperature, it takes a relatively long time for brazing. This increases probability of coarsening of crystal grains and/or deformation due to creep in metal parts or jigs. Accordingly, it was required to fabricate the metal parts or jigs tougher for preventing such crystal grain coarsening and deformation.
As above mentioned, in a process utilizing solid phase reaction by active metal, the reaction develops first at a contact surface of the graphite and the active metal, and then the bonding area enlarges due to thermal deformation and diffusion. Accordingly, it is necessary to apply a pressure over approximately 100 grams per square centimeter and also high temperature to the area along which the graphite is to be bonded to the metal, and further it is necessary to continue such brazing operation for a long time. For instance, the aforementioned Japanese Unexamined Patent Public Disclosure No. 62-21768 teaches that it needs a high temperature and a long time are required, specifically the temperature of 1,400 degrees centigrade for 90 hours, for bonding the graphite with the metal.
Such temperature is equal to or higher than a melting point, or higher than two thirds of the melting point of an ordinary industrial metalic material such as oxygen free copper, copper-nickel alloy, stainless steel and iron-nickel alloys. In addition, since the metalic materials are heated for long hours, melting of the metal, recrystallization of metal crystals and coarsening of crystal grains are remarkable, thereby remarkably decreasing the mechanical strength of the resultants.
In the case that the above mentioned process is to be applied to parts for use of an electron tube, there arise problems that metal parts are deformed due to high temperature creep; and that brazing materials in particular silver and gold develop into an alloy to thereby cause erosion of the brazing materials. This results in inaccurate dimensions and higher probability of occurrence of inferior sealing.
In the process disclosed in the above mentioned Japanese Unexamined Patent Public Disclosure No. 63-310778, in which a surface of a graphite member is in advance metalized with Ti--Cu--Ag brazing material, it takes relatively short time for brazing since the surface is metalized. However, in the case where brazing materials are to be used for areas to be exposed to electron beams or high temperature gas plasma, such as a collector of an electron tube and a gas laser tube, the metalized surface deteriorates inversely to the amount of the secondary electron emission and plasma-resisting property to a degree worse than the graphite.